In the Long Term Evolution (LTE) system, the Single Carrier Frequency Division Multiple Access (SC-FDMA) technique is expected to be used for uplink (UL) transmission, i.e., from a User Equipment (UE) to an evolved NodeB (eNB).
FIG. 1 shows a UL subframe structure (normal Cyclic Prefix (CP), no sounding reference signal) in the LTE system. As shown, one UL subframe includes two slots each containing 7 Orthogonal Frequency Division Multiplexing (OFDM) symbols. In the UL subframe, symbols #3 and #10 are UL reference symbols, or Demodulation Reference Signals (DMRSs), and the remaining symbols are data symbols. In order for the receiver (e.g., an eNB) to demodulate and decode the UL subframe, it is desired that the reference symbols and the data symbols have the same transmit power.
In the 3rd Generation Partnership Project (3GPP) LTE Release 11, a UE may have more than one UL transmission, e.g., on different carriers in a Carrier Aggregation (CA) scenario or to different access points (e.g., eNBs or Radio Remote Units (RRUs)) in a Dual Connectivity (DC) scenario. The UE may receive different Timing Advance (TA) commands for adjustment of its transmission timing for the respective UL transmissions. In this case, these UL transmissions may overlap each other.
FIG. 2 shows an example of overlap of UL transmissions. In this example, a UE has two UL transmissions, denoted as UL1 and UL2, respectively. As shown in FIG. 2, the last symbol (symbol #13) of the subframe #0 in UL1 overlaps the first symbol (symbol #0) of the subframe #1 in UL2, e.g., due to different TAs. In the overlapped portion, the transmit power of UL1 and/or UL2 may be forced to be reduced (depending on resource allocations to UL1 and UL2), such that the total transmit power in the overlapped portion will not exceed a preconfigured maximum transmit power of the UE. In the following, the subframe #1 in UL2 will be discussed as an example, without loss of generality.
FIG. 3 shows the subframe #1 in UL2 of FIG. 2, in which symbol #0 contains the overlapped portion. It is assumed here that UL1 is mute in the subframe #1. In this case, only the overlapped portion in the subframe #1 of UL2 has its transmit power reduced due to the limit of the maximum transmit power, while the transmit powers of the non-overlapped portion of symbol #0 and the remaining symbols of the subframe #1 remain unchanged. That is, the transmit power of the overlapped portion is scaled down relative to that of the DMRSs. In other words, the subframe is distorted.
FIG. 4 shows an example of a 16QAM (Quadrature Amplitude Modulation) constellation. If this is the original, non-scaled modulation constellation for the symbols #0-13 shown in FIG. 3, assuming a scaling factor of 5 dB, the modulation constellation for the overlapped portion will become distorted as shown in FIG. 5.
After receiving this subframe, an eNB first applies Fast Fourier Transform (FFT) to the received subframe, extracts OFDM symbols in the frequency domain, and obtains a channel estimation based on the DMRSs in symbols #3 and #10. Then, it applies frequency domain equalization and Inverse Discrete Fourier Transform (IDFT) to the OFDM symbols based on the channel estimation, computes soft bits and finally decodes the symbols. Since the transmit power of the overlapped portion is scaled relative to that of the DMRSs, the demodulation/decoding performance of the overlapped portion may be severely degraded when the channel estimation based on the DRMSs is applied. In turn, the demodulation/decoding performance of the entire subframe would be severely degraded due to error propagation. Accordingly, the subframe would contain random Cyclic Redundancy Check (CRC) errors due to not only channel quality, but also the distortion, which results in unstable link adaptation.
There is thus a need for a solution for improving the demodulation/decoding performance when such distortion occurs.